Navigation devices which display a current position of a vehicle or search information on a route to a desired destination for guiding a driver of the vehicle have been widely used in recent years. In order to perform display of a current position, search of a route, and navigation guidance, such a navigation device is equipped with map data which is divided by scale size according to an area of a region represented by the map. The scale sizes (levels) of a map data are hierarchically arranged from, for example, an “overview level” of 10 km×10 km in area down to, for example, a “detailed level” of 2.5 km×2.5 km in area, or further down to a more detailed level.
The map data can be obtained from a recording medium such as a DVD (Digital Versatile Disc) and a HDD (Hard Disk Drive) or a recording device. Recently, the map data can also be downloaded into a hard disk via a communication line. Actually, however, a network of roads is ever changing according to opening or closing of a road or the like. The map data once obtained has to be frequently updated such that the latest road network situation is reflected.
Regarding the update of a map data as mentioned above, if a navigation device having a hard disk capable of obtaining a map data via a communication line is used, there has been disclosed a technique of updating a map data by acquiring only a differential map data which is a data on a portion requiring an update, from a map delivery server (see, for example, Patent Document 1). The technique intends to reduce a map data update time and communication cost because such a portion to be changed of the map data typically accounts for only a small percentage.
Another technique discloses that, in order to compatibly conduct a map data update in a hard disk and navigation guidance (a navigation processing), respective operations for the update and the navigation guidance are made independent of each other and are performed in parallel by setting a cache memory and making the cache memory hold the map data in a cache (see, for example, Patent Document 2).
Prior Art Documents
Patent Documents
    Patent Document 1: Japanese Laid-Open Patent Application, Publication No. 2004-198841    Patent Document 2: Japanese Laid-Open Patent Application, Publication No. 2005-258340